The invention relates to a structural steel that can be bent back (i.e., especially able to be statically and dynamically stressed in the area of a bending point that has been bent back) as well as to the reinforcing connection produced by using such a reinforcing steel.
In structural engineering, structural steels are used in varied ways including use as reinforcement for a variety of concrete structural elements. These structural steels can be provided with ribbing or shaping on their surface, which can be configured in a variety of ways to achieve a sufficient bonding in the concrete.
Although all structural steels approved for concrete construction must meet the so-called "bend-back" test (i.e., from the aspect of their alloy or material structure, they must be made so that during this test the respective reinforcing steel does not harden or become brittle upon bending or bending back in such a manner that with bending back or slight stresses a break occurs afterwards), it is regarded as out of the question to use as structural steels those that have been bent and then bent back again where greater static or dynamic stress occur or can be expected.
Besides the bending of structural steels (but without bending back) quite common in construction engineering, it is often advantageous from the aspect of building construction or work cycle to use structural steels so that they are first bent in a specific area and later again bent up or bent back. A typical example of this are the so-called "reinforcing connections" which are increasingly used today, where to a first concrete structural element that is to be constructed (e.g. to a first concrete wall to be constructed) another concrete structural element (e.g., another concrete wall) is to be connected. In this case, the reinforcing steel, first bent and then bent back, or reinforcing bars, bent and then bent back, form the connecting reinforcement between the two concrete structural elements. These reinforcing connections, which make passing the connecting reinforcement through the form of the first concrete structural element constructed unnecessary, basically consist of a holding element, which can exhibit varied configuration and by which in each case the reinforcing bars, formed from a length of a reinforcing steel and provided with a corresponding ribbing or shaping, project out with a first partial length (anchoring area). With a second partial length (connecting area or part), bent substantially at right angles to the first partial length, the reinforcing bars are placed, in a covered manner, inside the holding element. Such a reinforcing connection is inserted into the concrete form for the first concrete structural element to be constructed so that the anchoring areas of the reinforcing bars are embedded in the concrete of the first concrete structural element constructed and the connection parts of the reinforcing bars are inside the holding element close to the form wall. After removal of the first constructed concrete structural element from the form and before concreting the concrete structural element to be connected, the connecting parts of the reinforcing bars are exposed and bent upward with a suitable tool, so that the connecting part, bent up or back, can be embedded in the concrete of the concrete structural element to be connected and thus form the connecting reinforcement on the transition area.
Despite bending (in making the reinforcing connection), as well as the subsequent bending up and back (in using the reinforcing connection) to achieve to some extent satisfactory results in regard to the carrying capacity of the connecting reinforcement, special heat-treated structural steels as reinforcing bars and special tools for bending the connecting parts upward have already been proposed. Nevertheless, in the case of usual structural steels, especially in bending back, microcracks in the structural steel cannot be avoided. Such microcracks decisively reduce the fatigue limit of the reinforcing steel, so that with all known structural steels, after bending and bending back, only relatively low fatigue limits on the order of 80 n/mm.sup.2 can be achieved. Because of this low fatigue limit, structural steels which have been bent and bent back are often used only where special stresses in the construction are not to be expected. This described problem is particularly serious if structural steels with relatively large diameters (for example, on the order of 6-16 mm) are necessary. For example, where an effort is made to obtain as small a radius of curvature or bending on the bending and bending back area to reduce the overall height of the holding element of a reinforcing connection or for other reasons.